Systems and methods for removing gaseous pollutants from a gas stream

ABSTRACT

Horizontal gas-liquid scrubbing systems and associated gas scrubbing methodologies are provided. In one embodiment, a horizontal duct scrubbing system includes a horizontally disposed housing having a waste gas inlet and a treated gas outlet, a liquid inlet manifold disposed within the horizontally disposed housing, the liquid inlet manifold comprising a plurality of nozzles oriented to spray a scrubbing liquor co-current to the flow of a gas stream flowing through the horizontally disposed housing, and a demister located proximal the treated gas outlet, where the horizontally disposed housing is substantially free of flow deflection members between the liquid inlet manifold and the demister.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/640,634, entitled “SYSTEMS AND METHODS FOR REMOVING GASEOUSPOLLUTANTS FROM A GAS STREAM”, filed Dec. 17, 2009, now U.S. Pat. No.______, which is a continuation of U.S. patent application Ser. No.11/868,904, entitled “SYSTEMS AND METHODS FOR REMOVING GASEOUSPOLLUTANTS FROM A GAS STREAM”, filed Oct. 8, 2007, now U.S. Pat. No.7,645,430. Each of the above identified U.S. patent applications isincorporated herein by reference in its entirety. The presentapplication is also related to PCT Patent Application No.PCT/US2008/076684, entitled “SYSTEMS AND METHODS FOR REMOVING GASEOUSPOLLUTANTS FROM A GAS STREAM”, and filed Sep. 17, 2008.

FIELD OF THE INVENTION

The present invention relates to systems and methods for removinggaseous pollutants, such as sulfur dioxide, from a waste gas stream(e.g., an industrial waste gas) via a horizontal gas-liquid scrubber.

BACKGROUND OF THE INVENTION

Gases containing pollutants, such as sulfur dioxide, are produced inmany industrial processes. One method of removing gaseous pollutantsfrom such gases involves the use of vertical scrubbing towers. Suchvertical scrubbing towers may be expensive to operate given the capitalcost and operating requirements of such towers. Horizontal scrubbers arealso known. For example, U.S. Pat. No. 5,403,568 to Stowe, Jr. disclosesa horizontal scrubber specifically for control of sulfur dioxideemissions and employs an aqueous scrubbing medium that flows parallel tothe gas flow. Stowe, Jr. requires the use of flow deflection members toimpede the flow of gases through the scrubber to cause a pressure dropin the scrubber and provide additional gas-liquid contact.

SUMMARY OF THE INVENTION

Broadly, the present invention relates to horizontal gas-liquidscrubbers for removing gaseous pollutants from gases, and systems andmethods relating to the same. The scrubbers are generally substantiallyfree of flow deflection members between a liquid inlet manifold and ademister downstream of the liquid inlet manifold. The scrubbers may beable to achieve at least about 71 vol. % sulfur dioxide removal at anL/G ratio of less than 20.

In one aspect, systems for removing sulfur from gases are provided. Inone approach, the system is a horizontal duct scrubbing system. In oneembodiment, the horizontal duct scrubbing system includes a horizontallydisposed housing, a liquid inlet manifold disposed within thehorizontally disposed housing, and a demister disposed within thehousing, where the horizontally disposed housing is substantially freeof flow deflection members between the liquid inlet manifold and thedemister. The housing includes a waste gas inlet and a treated gasoutlet. The demister may be located proximal the treated gas outlet. Theliquid inlet manifold may include a plurality of nozzles oriented tospray a scrubbing liquor co-current to the flow of a gas stream flowingthrough the horizontally disposed housing. In one embodiment, the liquidinlet manifold includes at least one lance (e.g., a plurality of lances)for supplying scrubbing liquor to the plurality of nozzles, and each ofthe lances includes at least one nozzle. In one embodiment, the liquidinlet manifold is a first liquid inlet manifold, and the system furtherincludes at least a second liquid inlet manifold disposed within thehorizontally disposed housing between the first liquid inlet manifoldand the demister, where the second liquid inlet manifold comprises aplurality of nozzles oriented to spray a scrubbing liquor co-current tothe flow of the gas stream.

In one embodiment, the horizontal duct scrubbing system is capable ofremoving at least 71 vol. % sulfur dioxide from an inlet gas. In oneembodiment, the horizontal duct scrubbing system is capable of removingat least 80 vol. % sulfur dioxide from the gas stream. In otherembodiments, the horizontal duct scrubbing system is capable of removingat least 90 vol. % or even 96 vol. % sulfur dioxide from the gas stream.In one embodiment, the inlet gas comprises not greater than 2000 ppm ofSO₂. In one embodiment, the system is capable of such sulfur removalrates at an L/G ratio of less than 20. In one embodiment, the horizontalduct scrubbing system achieves such removal rates at an L/G ratio of atleast about 5. In one embodiment, the horizontal duct scrubbing systemachieves such removal rates at an L/G ratio of not greater than about18. In one embodiment, the system includes a plurality of liquid inletmanifolds and the horizontal duct scrubbing system operates at an L/Gratio of less than 20 per liquid inlet manifold.

In one embodiment, sulfur dioxide is removed from the gas stream. Inother embodiments, the gas stream comprises at least one of NOx, HF,HCl, and SO₃, and the system is capable of removing at least some of theNOx, HF, HCl, and SO₃ from the gas stream.

In another aspect, methods of removing gaseous pollutants from a gas areprovided. In one approach, a method includes the steps of (i) flowing agas stream comprising gaseous pollutants into a horizontally disposedhousing via a waste gas inlet, the horizontally disposed housingcomprising the waste gas inlet at one end, a treated gas outlet at anopposite end, a liquid inlet manifold proximal the waste gas inlet and ademister proximal the treated gas outlet, where the horizontallydisposed housing is substantially free of flow deflection membersbetween the liquid inlet manifold and the demister, thereby defining asubstantially unrestricted gas-liquid contacting zone between the liquidinlet manifold and the demister, (ii) passing the gas stream through thesubstantially unrestricted gas-liquid contacting zone, (iii) spraying,concomitant to the passing step, a scrubbing liquor through thesubstantially unrestricted gas-liquid contacting zone and co-current tothe flow of the gas stream, thereby contacting the gas stream with thescrubbing liquor, and (iv) removing at least some of the gaseouspollutants from the gas stream during the contacting, where the gaseouspollutants comprise sulfur dioxide, and where after the removing step,at least about 71 vol. % sulfur dioxide is removed from the gas stream.In one embodiment, after the removing step, at least about 80 vol. %sulfur dioxide is removed from the gas stream. In other embodiments,after the removing step, at least about 90 vol. %, or even 96 vol. % ofsulfur dioxide is removed from the gas stream.

In one embodiment, the sulfur dioxide concentration of the gas streamprior to the passing step is not greater than 2000 ppm. In oneembodiment, a method includes operating the scrubbing system at an L/Gratio of less than 20 (e.g., during the flowing, passing, sprayingand/or removing steps). In one embodiment, the L/G ratio is at leastabout 5. In one embodiment, the L/G ratio is not greater than about 18.In one embodiment, during the passing step the gas stream has a speed ofat least about 12 feet per second. In one embodiment, during the passingstep the gas stream has a speed of not greater than about 45 feet persecond. In one embodiment, the pressure drop of the gas stream betweenthe waste gas inlet and the treated gas outlet is not greater than 6inches of water.

In one embodiment, sulfur dioxide is removed during the removing step.In other embodiments, one or more of NOx, HF, HCl, and SO₃ are includedin the gas stream, and the removing step comprises removing at leastsome of the NOx, HF, HCl, and SO₃ from the gas stream.

These and other aspects, advantages, and novel features of the inventionare set forth in part in the description that follows and will becomeapparent to those skilled in the art upon examination of the followingdescription and figures, or may be learned by practicing the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of one embodiment of a horizontal gas-liquidscrubbing system.

FIG. 2 is a partial cut-away view of the horizontal gas-liquid scrubbingsystem of FIG. 1 as viewed looking down the housing toward the gasinlet.

FIG. 3 is a schematic illustration of one embodiment of an in-ducthorizontal gas-liquid scrubbing system.

FIG. 4 is a graph illustrating sulfur dioxide removal as a function ofliquid-to-gas flow rate ratios for a horizontal gas-liquid scrubbingsystem utilizing a 50% contactor as compared to a horizontal gas-liquidscrubbing system that is free from flow deflection members therein.

FIG. 5 is a graph illustrating sulfur dioxide removal as a function ofalkalinity for a horizontal gas-liquid scrubbing system utilizing a 50%contactor as compared to a horizontal gas-liquid scrubbing system thatis free from flow deflection members therein.

FIG. 6 is a graph illustrating sulfur dioxide removal as a function ofdistance between nozzles and a demister.

FIG. 7 is a graph illustrating sulfur dioxide removal as a function ofalkalinity for a variety of housing sizes.

FIGS. 8 a-8 c are schematic illustrations of embodiments of nozzleplacement configurations.

FIG. 9 a is a graph illustrating sulfur dioxide removal as a function ofalkalinity for the nozzle configurations of FIGS. 8 a and 8 b.

FIG. 9 b is a graph illustrating sulfur dioxide removal as a function ofL/G for the nozzle configurations of FIGS. 8 b and 8 c.

FIG. 10 is a graph illustrating sulfur dioxide removal as a function ofL/G for a variety of nozzle types.

FIG. 11 is a graph illustrating sulfur dioxide removal as a function ofL/G for a waste gas stream comprising 80 ppm SO₂.

FIG. 12 is a graph illustrating sulfur dioxide removal as a function ofL/G for a waste gas stream comprising 200 ppm SO₂.

FIG. 13 is a graph illustrating sulfur dioxide removal as a function ofL/G for waste gas streams comprising 425, 600, and 1200 ppm SO₂.

FIG. 14 is a graph illustrating sulfur dioxide removal as a function ofL/G for waste gas streams with and without partial humidification of theinlet gas.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the accompanying drawings, whichat least assist in illustrating various pertinent embodiments of thepresent invention. Referring now to FIGS. 1 and 2, one embodiment of ahorizontal gas-liquid scrubbing system is illustrated. In theillustrated embodiment, the horizontal gas-liquid scrubbing system 1includes a horizontally disposed housing 10 comprising a waste gas inlet12 for receiving a waste gas stream 24, and a treated gas outlet 14 fordischarging a treated gas stream 26. In the illustrated embodiment, agas stream GS flows from the waste gas inlet 12, through thehorizontally disposed housing 10, and exits via the treated gas outlet14. The horizontally disposed housing 10 further comprises a liquidinlet manifold 16 downstream of the waste gas inlet 12. The liquid inletmanifold 16 comprises at least one inlet lance 17, each inlet lance 17being at least partially hollow to facilitate liquid flow therethrough.Each inlet lance has at least one nozzle 18 coupled thereto, andsometimes a plurality of nozzles 18 (“nozzle(s)”) coupled thereto. Theplurality of nozzle(s) 18 are oriented within the horizontally disposedhousing 10 so as to facilitate spraying of a scrubbing mist co-currentto the flow of the gas stream GS. In this regard, the nozzle(s) 18 arefluidly interconnected to a scrubbing liquor supply (not illustrated)via the inlet lances 17 of the liquid inlet manifold 16. Thehorizontally disposed housing 10 further includes a demister 20downstream of the liquid inlet manifold 16. The demister 20 is adaptedto remove entrained liquid droplets from the gas stream GS before thegas stream GS exits the horizontally disposed housing 10 as treated gasstream 26 via the treated gas outlet 14. The removed liquid exits thehorizontally disposed housing 10 via liquid outlets 22.

The horizontally disposed housing 10 is substantially free of flowdeflection members between the liquid inlet manifold 16 and the demister20, thereby defining a substantially unrestricted gas-liquid contactingzone 28. The present inventors have found that, even in the absence offlow defection members in the gas-liquid contacting zone 28, substantialremoval of gaseous pollutants in the gas stream GS may be facilitatedand with a relatively small footprint. As used herein, “gaseouspollutants” means any undesirable gaseous species contained in the GS.In one embodiment, the gaseous pollutants are acidic as compared to thescrubbing liquor. In one embodiment, the gaseous pollutants comprisesulfur species, such as sulfur dioxide and sulfur trioxide. In oneembodiment, the gaseous pollutants comprise one or more of NOx (e.g.,NO, NO₂) and halogenated hydrogen compounds (e.g., HF, HCl). In oneembodiment, the gaseous pollutants comprise volatile organic compounds(“VOCs)”, such as hydrocarbons, aldehydes and keytones, to name a few.

In one embodiment, the inlet gas stream comprises SO₂. In thisembodiment, the system 1 may be capable of removing at least about 65vol. % SO₂ from the gas stream GS. In other embodiments, the system 1may be capable of removing as at least about 70 vol. % SO₂ from the gasstream GS (e.g., at least about 71 vol. % SO₂), such as at least about75 vol. % SO2, or even at least about 80 vol. % SO₂, or even at leastabout 85 vol. % SO₂, or even at least about 90 vol. % SO₂, or even atleast about 95 vol. % SO₂ (e.g., at least about 96 vol. % SO₂).

The SO₂ removal rates may be achieved over a relatively large range ofSO₂ inlet concentrations. In one embodiment, the inlet concentration ofSO₂ in the gas stream GS is not greater than 2000 ppm of SO₂. In otherembodiments, the SO2 concentration is not greater than about 1500 ppm,such as not greater than about 1000 ppm, or even not greater than about500 ppm, or even not greater than about 200 ppm, or even not greaterthan about 100 ppm, or even not greater than about 80 ppm, or even notgreater than about 50 ppm, or even not greater than about 35 ppm.

The above SO₂ removal rates may be achieved with relatively small L/Gratios. As used herein, L/G means the average gallons per minute ofscrubbing liquor flow (“L”) into the liquid inlet manifold 16, asmeasured proximal the inlet to the liquid inlet manifold 16, per 1000actual cubic feet per minute (ACFM) of water saturated gas stream flowof the gas stream (G″). “G” may be measured directly or may becalculated, for example, by measuring the total flow rate of the gasstream proximal the waste gas inlet 12 and adjusting for evaporation ofwater and temperature reduction. In one embodiment, the L/G ratio isless than 20. In other embodiments, the L/G ratio is not greater thanabout 18, or even not greater than about 14, or even not greater thanabout 12. Generally, the L/G ratio is at least about 5. In someembodiments, the L/G ratio is at least about 8, or even at least about10.

As noted, the nozzle(s) 18 are fluidly interconnected within the liquidinlet manifold 16. The nozzle(s) 18 are operable to inject scrubbingliquor into the housing 10 to produce a scrubbing mist that flowsco-current to the gas stream GS. Thus, the nozzle(s) 18 are generallyconfigured with their ejection end disposed towards the demister 20. Thenozzle(s) 18 are generally operable to produce the scrubbing mist fromthe scrubbing liquor. The nozzle(s) 18 may be adapted to spray thescrubbing liquor in any suitable spray pattern to form the scrubbingmist within the housing. It is generally desirable for the nozzle(s) 18to produce a scrubbing mist that substantially covers a cross-sectionalarea of the horizontally disposed housing 10. Thus, the nozzle(s) 18 mayemploy a full cone spray pattern, a hollow cone spray pattern, a flatspray pattern, or any combination thereof.

The relative distance between the each of the nozzle(s) 18 may betailored so that the scrubbing mist from one nozzle overlaps with thescrubbing mist of one or more adjacent nozzle(s) 18. Hence, thescrubbing mist may substantially cover the cross-sectional area of thehorizontally disposed housing 10. The momentum of the scrubbing mistexiting the nozzles may be tailored so that appreciable amounts of thescrubbing mist remain airborne across a substantial length of thegas-liquid contacting zone 28. Thus, appreciable amounts of thescrubbing mist may substantially cover the cross-sectional area of thehorizontally disposed housing 10 and over a substantial length of thegas-liquid contacting zone 28, thereby providing intimate contactbetween the gas stream GS and the scrubbing mist in the substantiallyunrestricted gas-liquid contacting zone 28. Such intimate contact mayfacilitate reaction of gaseous pollutants in the gas stream GS withscrubbing agents in the scrubbing mist. In turn, high gaseous pollutantremoval rates may be realized.

Any number of nozzle(s) 18 with any number of spray patterns and/ordroplet sizes and in any combination may be utilized with the liquidinlet manifold 16 to facilitate operation of the horizontal gas-liquidscrubbing system 1. By way of primary illustration, the nozzle(s) 18 mayall comprise the same scrubbing mist generation capability (e.g., thesame spray pattern capability, momentum production capability, and/ordroplet size capability). By way of secondary illustration, a first setof nozzle(s) may comprise a first mist generation capability (e.g., afirst spray pattern, a first momentum production capability and/or afirst droplet size capability), and a second set of nozzle(s) maycomprise a second mist generation capability (e.g., a second spraypattern, a second momentum production capability and/or a second dropletsize range). In turn, other additional nozzle sets (e.g., third, fourth,etc.) may be utilized. Furthermore, any number of nozzles may beutilized per lance 17 of the liquid inlet manifold 16. In theillustrated embodiment, three nozzles 18 per lance 17 of the liquidinlet manifold 16 are utilized. However, other configurations may alsobe utilized. Furthermore, adjacent nozzle(s) 18 may be similarlyoriented in the up, down and/or side-to-side directions (e.g., sharingthe same vertical and/or horizontal plane), as illustrated, or adjacentnozzle(s) 18 may be offset relative to one or more other nozzle(s) 18.

In one embodiment, the nozzles are spiral jet nozzles. The spiral jetnozzles may be full cone or hollow cone nozzles. The spiral jet nozzlesmay have a 60-degree or 90-degree spray angle. The spiral jet nozzlesmay eject liquid at a pressure in the range of from about 40 psig toabout 65 psig. The spiral jet nozzles may have a liquid flow capacity inthe range of from about 15 gpm to about 50 gpm. The spiral jet nozzlesmay be constructed from durable materials so as to increase usagelifetime. In this regard, ceramics or other wear resistant materials,such as stainless steel or silicon carbide, may be used to construct thenozzles.

In general, the nozzle(s) 18 should be arranged to lower or restrict theamount of scrubbing mist that contacts the walls of the housing 10. Inthis regard, the housing 10 should generally be sized to lower orrestrict the amount of scrubbing mist contacting its walls. In anyevent, the cross-sectional area should not be so large that the pressuredrop across the housing 10 is not excessive.

The substantially unrestricted gas-liquid contacting zone 28 mayfacilitate low pressure drop across the horizontally disposed housing10. Pressure drop is a function of many variables, but it is anticipatedthat, in some embodiments, the pressure drop across the housing 10 maybe not greater than 8 inches of H₂O. In one embodiment, the pressuredrop is not greater than 6 inches of H₂O. In other embodiments, thepressure drop across the housing is not greater than 4 inches of water,or even not greater than 2 inches of H₂O, or even not greater than 1.5inches of H₂O, as measured at gas stream GS flow rates of between about12 feet per second and 45 feet per second.

As noted, the horizontally disposed housing 10 is substantially free offlow deflection members between the liquid inlet manifold 16 anddemister 20, which defines a substantially unrestricted gas-liquidcontacting zone 28. The length of the gas-liquid contacting zone 28 isapplication dependent, and may be a function of, for example, the flowrate of the gas stream GS through the housing, the inlet sulfurconcentration, the cross-sectional area of the housing, and/or theconcentration/alkalinity of the scrubbing liquor, to name a few.Generally, the length of the gas-liquid contacting zone 28 should belong enough to facilitate sufficient interaction between the gas streamGS and the scrubbing liquor so that suitable SO₂ removal rates arefacilitated (e.g., at least about 90 vol. % SO₂), but with restrictedpressure drop.

The demister 20 may be any suitable liquid removal apparatus capable ofremoving liquids from the gas stream GS within the applicable speed flowrates ranges. One particularly useful demister 20 for gas streams havinga speed in the range of about 12 to about 27 feet per second is a 2-passchevron-type mist reducer/eliminator.

The inlet gas stream GS may have a velocity suited to facilitate removalof sulfur species during its residence time within the scrubbing system1. In one embodiment, the gas stream has a bulk gas velocity (averageacross the housing) in the range of from about 10 feet per second (fps)to about 45 fps. In one embodiment, the gas stream has a bulk gasvelocity in the range of from about 22 fps to about 27 fps. Gas streamvelocities of 27 fps or less may be compatible with a 2-passchevron-type mist eliminator. Other type demisters may be used when thegas stream has higher bulk gas velocities, or expansion zones may beused to lower the gas stream bulk velocity, as described below.

In the illustrated embodiment, the housing 10 has a generally equalcross-sectional area. However, in other embodiments (not illustrated)the housing 10 may have varying cross-sectional areas. For example, afirst portion of the housing 10 may have a first cross-sectional areaand a second portion of the housing 10 may have a second cross-sectionalarea that is different than the first cross-sectional area. In oneembodiment, the second cross-sectional area may be larger than the firstcross-sectional area. In this embodiment, the second cross-sectionalarea may be considered an expansion zone, which may reduce the gasvelocity. In one approach (not illustrated), an expansion zone isincluded within the housing to facilitate reduction of the inlet gasfrom a first gas flow rate (e.g., greater than about 27 fps) to a secondgas flow rate (e.g., not greater than about 27 fps). Thus, in someinstances, the inlet velocity of the gas stream GS may be higher thanthe velocity of the gas stream proximal the demister. In some instances,the velocity of the gas stream may be as high as about 45 fps and theexpansion zone may be provided within the housing to facilitatereduction of the gas stream velocity prior to its contacting thedemister. In other embodiments, bulk velocity of the gas stream is inexcess of 27 fps and no expansion zone is utilized, but a compatibledemister is used.

The scrubbing liquor may be any liquid adapted to produce a scrubbingmist via the nozzle(s) 18 and to remove sulfur species from the wastegas stream 24 via interaction therewith (e.g., viareaction/absorption/entrainment). The misted scrubbing liquor may becapable of removing any of the above-described gaseous pollutants priorto the gas stream GS exiting the horizontally disposed housing 10 astreated gas stream 26, such as any of the above-described sulfur dioxideremoval rates. The misted scrubbing liquor should be capable of removingsuch gaseous pollutants at relatively low L/G ratios, such as any of theabove described L/G ratios. In one embodiment, the scrubbing liquor isan alkali feed liquid, such as a sodium-based liquid and may comprise,for example, sodium sulfite. In one embodiment, caustic and/or soda ashis used to produce the sodium sulfite of the scrubbing liquor. In otherembodiments, the scrubbing liquor may comprise other alkali liquids,such as one or more of seawater, lime-enhanced seawater,magnesium-promoted lime, and sodium-promoted lime, to name a few. Thus,in some embodiments, the horizontal gas-liquid scrubbing system may beintegrated with existing water containing/treatment systems to producethe scrubbing liquor and/or regenerate spent scrubbing liquor.

In one embodiment, the scrubbing liquor is sodium-based and isregenerable via a dilute mode dual alkali methodology or a concentratedmode dual alkali methodology, where a first reagent (e.g., sodiumsulfite species) is utilized to remove sulfur dioxides from the wastegas stream 24. In turn, a second reagent (e.g., slaked lime) is utilizedto regenerate waste liquids for use as the scrubbing liquor.

For a once-through system, the scrubbing liquor may have an alkalinitythat facilitates removal of sulfur dioxides from the waste gas stream24. The alkalinity of the scrubbing liquor varies by application and isgenerally related to the inlet SO₂ and CO₂ concentration. In oneembodiment, such as for waste gas streams comprising not greater than 80ppm of SO₂, the alkalinity of the scrubbing liquor may be at least about300 ppm as CaCO₃, but not greater than about 500 ppm as CaCO₃. Inanother embodiment, such as for waste gas streams comprising an inletconcentration of not greater than 200 ppm SO₂, the alkalinity of thescrubbing liquor may be at least about 750 ppm as CaCO₃, but not greaterthan about 1200 ppm as CaCO₃. Similarly, the pH of the scrubbing liquormay be tailored relative to the inlet sulfur concentration. Alkalinityand pH may be adjusted, as is known in the art, for use with waste gasstreams employing greater than 200 ppm SO₂.

The horizontal gas-liquid scrubbing system 1 may be utilized in avariety of applications. In one embodiment, the horizontal gas-liquidscrubbing system 1 is employed in an in-duct setting, where the wastegas inlet 12 and treated gas outlet are directly interconnected to awaste gas duct of an industrial facility. One embodiment of such aconfiguration is illustrated in FIG. 3. In the illustrated embodiment,waste gas 24 from an industrial facility (not illustrated) exits suchindustrial facility via ductwork 50. The horizontal gas-liquid scrubbingsystem 1 is fluidly interconnected to the ductwork 50 via piping 51 andgas inlet 12. As described above, such waste gas 24 may be treated viathe horizontal gas-liquid scrubbing system 1, and treated gas 26 may beexhausted via the treated gas outlet 14. In this regard, the system 1may include one or more transition zones (described above) to obtainsuitable gas stream flow rates and/or may include suitable demisters. Inturn, the treated gas outlet 14 may be fluidly interconnected withexhaust ductwork 54 via piping 53. Such treated gas 26 may be exhaustedto the atmosphere. Hence, in one embodiment, ductwork 50 and exhaustductwork 54 are preexisting prior to installation of the horizontalgas-liquid scrubbing system 1, and a channel 52 interconnecting ductwork50 and exhaust ductwork 54 may be removed to facilitate incorporation ofthe horizontal gas-liquid scrubbing system 1. Thus, in some embodiments,the horizontal gas-liquid scrubbing system 1 may be added to existingfacilities with relatively minor adjustments to existing waste gastreatment systems. In other embodiments, the horizontal gas-liquidscrubbing system is used in non-retrofit applications, such as newlyconstructed gas scrubbers.

The waste gas stream 24 may be any waste gas stream requiring removal ofgaseous pollutants therefrom. For example, the waste gas stream may be awaste gas from an aluminum smelting facility, industrial steam boilers,or a steel manufacturing facility, to name a few. In one approach, thewaste gas stream may be a concentrated waste gas stream from an aluminumsmelting facility, such as a concentrated waste gas stream from a dryscrubbing concentration operation, such as by that disclosed in PCTApplication No. PCT/FR2006/000415, the contents of which areincorporated herein by reference in its entirety.

In one embodiment, the waste gas stream comprises a concentration of notgreater than about 2000 ppm SO₂, on average. In a particular embodiment,the waste gas stream comprises not greater than about 200 ppm SO₂, onaverage (e.g., a waste gas from an aluminum smelting facility), and thescrubbing liquor comprises soda ash. In another embodiment, the wastegas stream comprises not greater than 2000 ppm SO₂, on average, and adilute mode dual alkali, a concentrated mode dual alkali, asodium-promoted lime, or a magnesium-promoted lime scrubbing methodologymay be utilized.

The temperature and humidity of the incoming waste gas stream 24 mayaffect the performance of the system 1. In one embodiment, a coolingwater spray upstream of the liquid inlet manifold 16 is used to at leastpartially humidify and/or cool the incoming waste gas stream, sometimesreferred to herein as pre-humidification. For example, and withreference to FIG. 1, the horizontal gas-liquid scrubbing system 1 mayoptionally comprise a cooling spray 40 disposed upstream of the liquidinlet manifold 16 for spraying water (or other suitable cooling medium)into the entering waste gas stream 24 for cooling such waste gas stream24. The cooling spray 40 may spray cooling water in a countercurrent orco-current fashion relative to the waste gas stream 24 to facilitatesuch cooling. The volumetric flow rate of such cooling liquid spray isgenerally application dependent. The use of pre-humidification mayresult in at least about a 2 vol. % increase in SO₂ removal, or even atleast about a 3% vol. increase, or even about a 4 vol. % increase. Insome embodiments, using pre-humidification may allow the system 1 tooperate at lower L/G ratios while still achieving suitable SO₂ removalrates.

As noted above, the system may remove gaseous pollutants other than SO₂from a waste gas stream. For example, the system may remove one or moreof NOx, HCl, HF, SO₃, and VOCs, to name a few. Various scrubbing liquorsmay be employed in this regard. In any event, the scrubbing liquor mayhave a basic pH relative to the gaseous pollutants (except the VOCs) soas to facilitate their removal.

EXAMPLES Example 1 Evaluation of SO₂ Removal With and Without FlowDeflection Members

A horizontal gas-liquid scrubbing system similar to the one illustratedin FIG. 1 is produced in two configurations. The first configurationincludes a 50% open contactor between the nozzles and the demister. Thesecond configuration is free of flow deflection members between thenozzles and the demister. The demister is a 2-pass chevron-type misteliminator. Waste gas having an average SO₂ concentration of about 80ppm is provided to the system at a volumetric flow rate of between about3,500 and 5,000 ACFM, with an average velocity of about 22 feet persecond. Various nozzles types, including whirljet-type, also known asW-type nozzles, and sprayjet-type, also known as BSL-type or pigtailnozzles, having a 90° spray angle and a hollow cone spray are utilizedduring the testing. A scrubbing liquor supply tank comprising sodiumsulfite and sodium carbonate is supplied to the nozzles at an L/G ratioin the range of from about 15 to about 18.

FIG. 4 illustrates the effect of SO₂ removal efficiency relative to L/Gratio utilizing both the first configuration (50% contactor) and thesecond configuration (no flow deflection members). FIG. 5 illustratesthe effect of SO₂ removal efficiency relative to alkalinity in both thefirst configuration (50% contactor) and the second configuration (noflow deflection members). Higher average SO₂ removal rates are realizedin the second configuration with no flow deflection members between thenozzles and demister.

Example 2 Evaluation of Distance Between Nozzles and Demister

A horizontal gas-liquid scrubbing system similar to the one illustratedin FIG. 1 is produced. Waste gas having an average SO₂ concentration ofabout 77 ppm and a flow rate of about 23 feet per second is provided tothe system. The L/G ratio is about 16. The demister is a 2-passchevron-type mist eliminator and is located about 20 feet downstream ofthe nozzles. The SO₂ concentration in the gas is measured between thenozzles and the demister, the results of which are illustrated in FIG.6. SO₂ removal efficiency increases dramatically at distances from 0feet to 8 feet between the demister and the nozzles, and levels outthereafter.

Example 3 Evaluation of Housing Size and Number of Nozzles

Various horizontal gas-liquid scrubbing systems similar to the oneillustrated in FIG. 1 are produced. The size of the housing and theamount of nozzles within the housing are varied. An inlet gas havingabout between about 100 ppm and 200 ppm SO₂ and a velocity of about 22fps is used. A sodium-based scrubbing liquor having an alkalinity in therange of from about 600 ppm as CaCO₃ to about 1400 ppm as CaCO₃ is used.The system is operated at an L/G ratio of about 17. As illustrated inFIG. 7, over 90 vol. % SO₂ removal is achieved in systems having alarger cross-sectional area and more spray nozzles at moderatealkalinity values (600-800 ppm) as CaCO₃, while 90 vol. % SO₂ removal isnot achieved in smaller housings with fewer nozzles, even at alkalinityvalues of 1000 ppm as CaCO₃ and higher. It is postulated that smallerhousings have greater “edge effects” in that a greater percentage of themist impinges on the walls of the housing, thereby reducing theeffective amount of mist available for interaction with the waste gasstream.

Example 4 Evaluation of Nozzle Array Configuration

A horizontal gas-liquid scrubbing system similar to the one illustratedin FIG. 1 is produced. The housing has a width of 18.5″ and a length of29.75″. A single bank of nozzles having a variety of nozzle arrangementsare utilized. These nozzle array configurations are illustrated in FIGS.8 a-8 c. A first array is configured as illustrated in FIG. 8 a andincludes a 3×3 array of BSJ9040 (hollow cone) nozzles. A second array isconfigured as illustrated in FIG. 8 a and includes a 3×3 array ofBSJ9040 (full cone) nozzles. A third array is configured as illustratedin FIG. 8 b and includes a 3×3 array of BSJ9040 (hollow cone) nozzles. Afourth array is configured as illustrated in FIG. 8 b and includes a 3×3array of BSJ9040 (full cone) nozzles. A fifth array is configured asillustrated in FIG. 8 c and includes a 3×1 array of BSJ90120 nozzles. Aninlet gas comprising about 100 ppm SO₂, and a sodium-based alkalinescrubbing liquor are used. The L/G ratio is varied and the gas inletvelocity is about 22 fps. As illustrated in FIGS. 9 a and 9 b, thenozzle array configuration of FIG. 8 b yields higher SO₂ removalefficiency than the nozzle array configuration of FIGS. 8 a and 8 c.Furthermore, the hollow cone nozzles realize superior performance thanthe full cone nozzles.

Example 5 Evaluation of Nozzle Types

A horizontal gas-liquid scrubbing system similar to the one illustratedin FIG. 1 is produced. The housing has a width of 27.625″ and a lengthof 27.625″. An inlet gas comprising about 100 ppm SO₂, and asodium-based alkaline scrubbing liquor are used. The L/G ratio is variedand the gas inlet velocity is about 22 fps. A variety of nozzles areused to test the SO₂ removal performance of the nozzles. Tested nozzlesincludes spiral jet nozzles, tangential jet nozzles and whirl jetnozzles. The spray angle is also varied in some nozzles (90° and 60°) aswell as the cone type (hollow cone and full cone). As illustrated inFIG. 10, the spiral jet, 90°, hollow cone nozzles perform the best overthe range of the varied L/G ratio.

Example 6 Evaluation of Various Inlet SO₂ Concentrations and ScrubbingLiquors

A horizontal gas-liquid scrubbing system similar to the one illustratedin FIG. 1 is produced. Various waste gas streams have a bulk gasvelocity of about 22 feet per second and SO₂ concentrations of fromabout 80 ppm to about 1200 ppm are treated with the system and at an L/Grange of from about 11 to about 18. Various scrubbing liquors are usedto treat the waste gas streams. In particular, in the 80 ppm and 200 ppmtests, a dilute mode dual alkali approach is utilized, and in the 425ppm, 600 ppm, and 1200 ppm tests, a concentrated mode dual alkaliapproach is utilized. The system consistently removes at least 85 vol. %SO₂, and often at least 90 vol. % SO₂, from the waste gases, and with anL/G of less than 18, and often less than 16. FIG. 11 illustrates theamount of sulfur dioxide removed as a function of L/G for a waste gasstream comprising 80 ppm SO₂. FIG. 12 illustrates the amount of sulfurdioxide removed as a function of L/G for a waste gas stream comprising200 ppm SO₂. FIG. 13 illustrates the amount of sulfur dioxide removed asa function of L/G for waste gas streams comprising 425 ppm, 600 ppm, and1200 ppm SO₂.

Example 7 Effect of Pre-Humidification of the Waste Gas Stream

A horizontal gas-liquid scrubbing system similar to the one illustratedin FIG. 1 is produced. A waste gas stream having a bulk gas velocity ofabout 21 feet per second and an SO₂ concentration of about 80 ppm istreated with the system at an L/G range of from about 10 to about 16. Ina first approach, the waste gas stream is treated withoutpre-humidifying the waste gas stream prior to treating the waste gasstream with the scrubbing liquor. In a second approach, the waste gasstream is pre-humidified prior to treating the waste gas stream with thescrubbing liquor. As illustrated in FIG. 14, the system removes fromabout 2.7 vol. % to about 4.0 vol. % more SO₂ using pre-humidification.

While the term “nozzles” has been used herein to describe one type ofmist generation device, such term has been used merely for illustrativepurposes, and it is expressly contemplated that other mist generationdevices may be utilized to produce a mist within the housing of thehorizontal gas-liquid scrubber, such as jets, ultrasonics and otherdevices. Furthermore, while various embodiments of the present inventionhave been described in detail, it is apparent that modifications andadaptations of those embodiments will occur to those skilled in the art.However, it is to be expressly understood that such modifications andadaptations are within the spirit and scope of the present invention.

1. A method for removing gaseous pollutants from a gas, the methodcomprising: (a) flowing a gas stream comprising gaseous pollutants intoa horizontally disposed housing via a waste gas inlet, the horizontallydisposed housing comprising the waste gas inlet at one end, a treatedgas outlet at an opposite end, a liquid inlet manifold proximal thewaste gas inlet and a demister proximal the treated gas outlet, whereinthe horizontally disposed housing is substantially free of flowdeflection members between the liquid inlet manifold and the demister,thereby defining a substantially unrestricted gas-liquid contacting zonebetween the liquid inlet manifold and the demister; (b) passing the gasstream through the substantially unrestricted gas-liquid contactingzone; (c) spraying, concomitant to the passing step, a scrubbing liquorthrough the substantially unrestricted gas-liquid contacting zone andco-current to the flow of the gas stream, thereby contacting the gasstream with the scrubbing liquor; (d) removing at least some of thegaseous pollutants from the gas stream during the contacting; (i)wherein the gaseous pollutants comprise at least one of (i) one or morehalogenated hydrogen compounds, and (ii) volatile organic compounds; and(e) operating, during the flowing, passing, spraying and removing steps,the horizontal duct scrubbing system at an L/G ratio of less than
 20. 2.The method of claim 1, wherein the L/G ratio is at least about
 5. 3. Themethod of claim 2, wherein the L/G ratio is not greater than about 18.4. The method of claim 1, wherein, during the passing step, the gasstream has a speed of at least about 12 feet per second.
 5. The methodof claim 4, wherein, during the passing step, the gas stream has a speedof not greater than about 45 feet per second.
 6. The method of claim 1,wherein, during the operating step, the pressure drop of the gas streambetween the waste gas inlet and the treated gas outlet is not greaterthan 6 inches of water.
 7. The method of claim 6, wherein the L/G ratiois not greater than about
 18. 8. The method of claim 7, wherein the L/Gratio is not greater than about
 14. 9. The method of claim 8, whereinthe L/G ratio is not greater than about
 12. 10. The method of claim 1,comprising: upstream of the liquid inlet manifold, at least partiallyhumidifying the gas stream.
 11. A horizontal duct scrubbing system forremoving gaseous pollutants from a gas, the horizontal duct scrubbingsystem comprising: (a) a horizontally disposed housing having a wastegas inlet for receiving a gas stream and a treated gas outlet fordischarging the gas stream; wherein the gas stream comprises at leastone of (i) one or more halogenated hydrogen compounds, and (ii) volatileorganic compounds; (b) a liquid inlet manifold disposed within thehorizontally disposed housing, the liquid inlet manifold comprising aplurality of nozzles oriented to spray a scrubbing liquor co-current tothe flow of the gas stream; (i) wherein the plurality of nozzles of theliquid inlet manifold are configured to spray the scrubbing liquor at anaverage flow rate (L) of less than 20 gallons per minute per 1000 actualcubic feet per minute (ACFM) of water saturated gas stream flow exitingthe treated gas outlet (G); and (c) a demister located proximal thetreated gas outlet, wherein the horizontally disposed housing issubstantially free of flow deflection members between the liquid inletmanifold and the demister.
 12. The system of claim 11, wherein theliquid inlet manifold comprises a plurality of lances for supplying thescrubbing liquor to the plurality of nozzles, wherein each of the lancescomprises at least one of the plurality of nozzles.
 13. The system ofclaim 12, wherein the liquid inlet manifold is a first liquid inletmanifold, and wherein the system further comprises: at least a secondliquid inlet manifold disposed between the first liquid inlet manifoldand the demister within the horizontally disposed housing, wherein thesecond liquid inlet manifold comprises a plurality of nozzles orientedto spray a scrubbing liquor co-current to the flow of the gas stream.